[tests] Add a test for SIGINT handling.
[tests] Add a test for SIGINT handling.

 - Aaaah, it's amazing what one can do with a little shell.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild